Refining apparatus or disc refiners of the type discussed above are used, inter alia, for highly concentrated refining, CTMP, TMP, fluffing and highly concentrated grinding of sack paper and other lignocellulose-containing material. They usually comprise two opposite refining discs rotatable in relation to each other, where often one refining disc is rotatable, a so called rotor, and the other refining disc is non-rotatable, a so called stator, but in some refining apparatus both refining discs are rotatably arranged. Refining discs in this type of refining apparatus are provided with exchangeable refining segments which build up the refining surfaces of the refining apparatus. The refining segments comprise bars and intermediate grooves. Grinding occurs between the two refining surfaces which are kept at a certain distance from each other, whereby a space, a so called refining gap, is provided between the refining surfaces.
If the refining surfaces were to come into contact with each other during operation, there is a risk of breakdown or at least wear on the refining surfaces, and with that a shortened operating performance. Furthermore, the degree of alignment between the refining surfaces has great significance regarding the quality of the ground material. When the degree of alignment between the refining surfaces is reduced, the quality of the ground material deteriorates. An accurate control of the refining gap and the alignment between the refining surfaces is thus of great importance.
One way of measuring the distance between the refining surfaces in a refining apparatus is disclosed in Swedish Patent No. 416,844, which discloses a device and a method for measuring the distance between two opposite surfaces, made of a magnetic conducting material, according to the reluctance method by means of a position sensor which is provided in one of the surfaces and is orientated in relation to the second surface so that the air gap between the surfaces is included in the circuit. The method comprises the use of a sensor having two windings provided around a core, which are supplied with current so that they work in opposite directions, and the currents are controlled so that the resulting magnetic flux through a direct current field meter located between the windings is always kept equal to zero, whereby the measurement result is attained by measuring the difference between the currents supplied to the windings. It is also known to make such a sensor displaceable from one of the surfaces towards the other surface, for position calibration.
Swedish Patent No. 463,396 discloses a device for indicating the axial contact position of the refining surfaces of two opposite refining discs rotatable in relation to each other and included in a disc refiner. A sensor sensing heat radiation is arranged to detect the heat radiation which arises through the friction as two refining surfaces contact each other during rotation in relation to each other. The sensor is positioned radially outside the refining discs.
Swedish Patent No. 454,189 describes a method for controlling the production of mechanical pulp in a refiner process, where lignocellulose-containing material in pieces is refined when passing through the refining gap between two opposite refining discs rotating in relation to each other. The vibrations of at least one of the refining discs are measured by means of an accelerometer provided in the refining disc and are transformed to vibration energy, which, together with one or several of the process variables: production, size of the refining gap and material concentration, is used for controlling the properties of the produced pulp. Further, Swedish Patent No. 454,189 discloses that the condition of the refining segment can also be established by the measured vibrational energy, which can be used for determining when it is time for exchanging refining segments, and different refining segment patterns and refining segment material can be compared.
British Patent No. 1,468,649 discloses a method for adjusting the refining surfaces included in a refining apparatus, so that these are parallel when grinding wood chips into pulp, which refining apparatus comprises a stationary refining disc and a rotatable refining disc, the stationary refining disc being attached to the frame of the refining apparatus by means of three fixing pins of which at least one is heatable for instance by an electric current, so that its length is variable to achieve parallelism between the refining surfaces of the refining discs. The method comprises the steps of continuously measuring the axial force between the refining surfaces and maintaining this force at its maximum by shortening or lengthening the length of the heatable fixing pin. This force is measured by measuring means provided on the rotation axis of the rotatable refining disc.
However, the method of British Patent No. 1,468,649 assumes that the material intended for grinding has a certain moisture content, which is defined as a dry matter content between about 15 and 40%, so that steam is generated between the refining surfaces, whereby this steam gives rise to the greater part of the pressure between the refining surfaces. The method is based on the conclusion that when the degree of parallelism between the refining surfaces reduces, the pressure between the refining surfaces is reduced, which pressure reduction can be measured as a reduction of the axial force. When the pressure is at its maximum, the degree of parallelism is considered to be maximized. However, this means that when the material intended for grinding changes, for example regarding type, size and dry matter content, or the temperature in the refining gap changes, the size of the maximum force attained when the degree of parallelism or alignment between the refining surfaces is at its maximum also changes.
One of the objects of the present invention is thus to provide a method and a device which more effectively control the alignment between two opposite refining surfaces rotatable in relation to each other, which refining surfaces are included in a refining apparatus, in relation to the prior art. Another object hereof is to provide a refining apparatus provided with such a device.